Polyethylene polymers are well known and are useful in many applications. In particular, linear polyethylene polymers possess properties which distinguish them from other polyethylene polymers, such as branched ethylene homopolymers commonly referred to as LDPE (low density polyethylene). Certain of these properties are described by Anderson et al, U.S. Pat. No. 4,076,698.
A particularly useful polymerization medium for producing polyethylene polymers is a gas phase process. Examples of such are given in U.S. Pat. Nos. 3,709,853; 4,003,712; 4,011,382; 4,302,566; 4,543,399; 4,882,400; 5,352,749 and 5,541,270 and Canadian Patent No. 991,798 and Belgian Patent No. 839,380.
Ziegler-Natta type catalyst systems for the polymerization of olefins are well known in the art and have been known at least since the issuance of U.S. Pat. No. 3,113,115. Thereafter, many patents have been issued relating to new or improved Ziegler-Natta type catalysts. Exemplary of such patents are U.S. Pat. Nos. 3,594,330; 3,676,415; 3,644,318; 3,917,575; 4,105,847; 4,148,754; 4,256,866; 4,298,713; 4,311,752; 4,363,904; 4,481,301 and U.S. Pat. No. Re. 33,683.
These patents disclose Ziegler-Natta type catalysts that are well known as typically consisting of a transition metal component and a co-catalyst that is typically an organoaluminum compound. Optionally used with the catalyst are activators such as halogenated hydrocarbons and activity modifiers such as electron donors.
The use of halogenated hydrocarbons with Ziegler-Natta type polymerization catalysts in the production of polyethylene is disclosed in U.S. Pat. No. 3,354,139 and European Patent Nos. EP 0 529 977 B1 and EP 0 703 246 A1. As disclosed, the halogenated hydrocarbons may reduce the rate of ethane formation, improve catalyst efficiency, or provide other effects. Typical of such halogenated hydrocarbons are monohalogen and polyhalogen substituted saturated or unsaturated aliphatic, alicyclic, or aromatic hydrocarbons having 1 to 12 carbon atoms. Exemplary aliphatic compounds include methyl chloride, methyl bromide, methyl iodide, methylene chloride, methylene bromide, methylene iodide, chloroform, bromoform, iodoform, carbon tetrachloride, carbon tetrabromide, carbon tetraiodide, ethyl chloride, ethyl bromide, 1,2-dichloroethane, 1,2-dibromoethane, methylchloroform, perchloroethylene and the like. Exemplary alicyclic compounds include chlorocyclopropane, tetrachlorocyclopentane and the like. Exemplary aromatic compounds include chlorobenzene, hexabromobenzene, benzotrichloride and the like. These compounds may be used individually or as mixtures thereof.
It is also well known, in the polymerization of olefins, particularly where Ziegler-Natta type catalysts are employed, to utilize, optionally, electron donors. Such electron donors often aid in increasing the efficiency of the catalyst and/or in controlling the stereospecificity of the polymer when an olefin, other than ethylene, is polymerized. Electron donors, typically known as Lewis Bases, when employed during the catalyst preparation step are referred to as internal electron donors. Electron donors are utilized other than during the catalyst preparation step are referred to as external electron donors. For example, the external electron donor may be added to the preformed catalyst, to the prepolymer, and/or to the polymerization medium.
The use of electron donors in the field of propylene polymerization is well known and is primarily used to reduce the atactic form of the polymer and increase the production of the isotactic polymers. The use of electron donors generally improves the productivity of the catalyst in the production of isotactic polypropylene. This is shown generally in U.S. Pat. No. 4,981,930.
In the field of ethylene polymerization, where ethylene constitutes at least about 70% by weight of the total monomers present in the polymer, electron donors are utilized to control the molecular weight distribution (MWD) of the polymer and the activity of the catalyst in the polymerization medium. Exemplary patents describing the use of internal electron donors in producing linear polyethylene are U.S. Pat. Nos. 3,917,575; 4,187,385, 4,256,866; 4,293,673; 4,296,223; U.S. Pat. No. Re. 33,683; U.S. Pat. Nos. 4,302,565; 4,302,566; and 5,470,812. The use of an external monoether electron donor, such as tetrahydrofuran (THF), to control molecular weight distribution is shown in U.S. Pat. No. 5,055,535; and the use of external electron donors to control the reactivity of catalyst particles is described in U.S. Pat. No. 5,410,002.
Illustrative examples of electron donors include carboxylic acids, carboxylic acid esters, alcohols, ethers, ketones, amines, amides, nitriles, aldehydes, thioethers, thioesters, carbonic esters, organosilicon compounds containing oxygen atoms, and phosphorus, arsenic or antimony compounds connected to an organic group through a carbon or oxygen atom.